. A textbook of botany for colleges and universities ... Botany. LEAVES 613 which consists of a rapid ascent of water between the stems or of a rapid filling of air spaces (or of both in Sphagnum and Leucobryum); the second phase is represented by a slow osmotic movement, as in root hairs. The water which is absorbed so rapidly byhchens and by cushion mosses, is lost more slowly by transpiration. Sphagnum being able to absorb as much in a minute as is lost by ordinary transpiration in a week. While some cushion mosses are mesophytic (as in Bar- tramia and some species of Dicranmn), others tend
. A textbook of botany for colleges and universities ... Botany. LEAVES 613 which consists of a rapid ascent of water between the stems or of a rapid filling of air spaces (or of both in Sphagnum and Leucobryum); the second phase is represented by a slow osmotic movement, as in root hairs. The water which is absorbed so rapidly byhchens and by cushion mosses, is lost more slowly by transpiration. Sphagnum being able to absorb as much in a minute as is lost by ordinary transpiration in a week. While some cushion mosses are mesophytic (as in Bar- tramia and some species of Dicranmn), others tend toward xerophytism (as in Leucobryum), and even Sphagnum may be called a bog xerophyte. Lichens usually are pronounced xerophytes, and (as with some mosses) absorption must be a com- paratively infrequent phenomenon (particularly since only liquid water can be absorbed in quan- tity), while exposure to transpiration is frequent.^ For aerial absorption to be advantageous in such plants, it must be accompanied by an ability to endure prolonged desiccation, an ability possessed by lichens and by some mosses in superlative Figs, goi, 902. — Aspects of Polytrichum commune, a xerophytic moss: 901, a leafy shoot, as seen when the water supply is adequate; 902, a simi- lar shoot that has been exposed to desiccation, the loss of water hav- ing caused the leaves to become appressed to the stem; when the base of such a shoot is placed in water, the leaves soon assume the position seen in Fig. 901. Vascular plants.—-The aerial leaves of ferns and seed plants are cutinized, and water absorption commonly is so slight as to be without significance.^ Wilted leaves, when placed in contact with water, absorb enough to enable them to recover their usual turgescence, but this phenom- enon probably is of little significance in nature. Water absorption has been predicated as a. function of many living leaf hairs, especially in xerophytes, largely, perhaps, because their water supply is
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectbotany, bookyear1910
